In composite structures, joints between adjacent structure components, such as fiber-reinforced sheets and/or reinforcing ribs, are often subject to stress concentrations which may cause crack initiation. At worst, such cracks may propagate through the structure and lead to failure.
A major cause of the stress concentrations are voids or gaps, which commonly are formed in connection with component junctions. The traditional remedy in this regard is to occupy the voids by introducing a material; a so-called radius filler.
During manufacture of composite structures from fiber-reinforced sheets, it is furthermore important that the individual sheets maintain a desired shape in the juncture regions. The forming of the sheets to the desired shape, and/or pressure applied in the manufacturing process, may force sheets to enter the juncture gaps, whereby the fibers of the sheets are misaligned and the bearing capacity of the structure decreased. Such misalignment can be counteracted by means of radius fillers, provided that the radius fillers are rigid, and thus provide backing, during manufacture of the composite structures.
Thus, radius fillers can be used in order to increase fatigue properties and structural strength of composite structures.
U.S. Pat. No. 5,650,229 A discloses a preformed radius filler which is adapted to fill a gap between plies of a composite article. The filler comprises unidirectional fibers and a stabilizer, which stabilizer is compatible with, and simultaneously curable with, a subsequently injected resin.
In U.S. Pat. No. 5,827,383 A, a radius filler comprising fibers and resin is used, and a joint between fiber reinforced sheets is further strengthened by means of pins.
US20030183067 A1 describes a fiber reinforced radius filler with a binding agent/tackifier solution.
A problem associated with prior radius fillers, is that they are relatively complex in regards to structure and manufacture. Further, known radius fillers present unsatisfying shapes and provide poor support to composite sheets during forming of the sheets.